When instruments using different frequencies are coupled to each other, it is necessary to convert a sampling frequency of the instrument on a sending side to that of the instrument on a receiving side.
As one of representative techniques on the sampling frequency conversion method, there has been a method in which output data is constructed by over-sampling input data by N times and by thinning out the over-sampled data by 1/M times (for example, Japanese Patent Laid-Open Publication No. Hei 6(1994)-120776, page 2, FIG. 5). M and N are integers. By this sampling frequency conversion method, the input data is converted to a sampling frequency multiplied by N/M.
However, if a sampling frequency ratio of the input data and the output data is not previously known, the above described sampling frequency conversion method is incapable of converting the sampling frequency and of being applied.
As the case where the sampling frequency ratio of the input data and the output data is not known, in an I/O system using a processor for a data processing, thought is a case where a clock for driving the I/O system and a clock for driving an instrument on a sending side to output data to the I/O system are asynchronous.
In such an I/O system, when a sampling frequency of the I/O system and a sampling frequency of the instrument on a sending side are different, a phase shift occurs between input data and output data. This phase shift is accumulated with passage of time, and a shift equivalent to one or more samples occurs between the input data and the output data. Data between the input data and the output data becomes discontinuous.